It is possible to convert renewable energy such as wind, wave, tidal energy or water current flows into electrical energy by using a turbine to drive the rotor of a generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the renewable-energy turbine will vary according to the speed of the wind or water current flows driving the blades of the turbine assembly. Matching of the variable voltage and frequency of the generator to the nominally constant voltage and frequency of the power network can be achieved by using a power converter.
The efficiency of a turbine is usually expressed by a Cp-Lambda curve. In a given cross section through a fluid flow, there is an amount of power present given by the flow rate and the kinetic energy per unit volume that it contains. Cp represents the efficiency of the turbine in converting this power into mechanical power. Lambda is the turbine blade tip speed divided by fluid flow rate.